JPH05281256A - Capacitive sensor - Google Patents

Abstract

PURPOSE:To improve reliability of a capacitive sensor by adding a failure diagnostic function. CONSTITUTION:A detecting part 10 consists of a movable electrode 13 supported by a beam 14 and having a weight function, and fixed weights 11 and 12 facing the movable electrode 13. Relating to diagnosis, the electrostatic diagnosis in which a high voltage occurring at a boosting circuit 9 is, through resistors 7 and 8, applied to the fixed electrodes 11 and 12, so as to forcedly oscillate the movable electrode 13 by electrostatic force, and a leakage current inspecting diagnosis in which switches 3 and 4 connect the fixed electrodes 11 and 12 to a power source and then an electrostatic capacitance detector 15 inspects the leakage current flowing from the fixed electrodes 11 and 12 to the movable electrode 13 are included. Since diagnosing the failure of capacitive sensor is possible, the reliability of the system using the capacitive sensor improves.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitive sensor, and more particularly to a highly reliable capacitive sensor capable of detecting a high frequency detection target from a weak DC.

[0002]

2. Description of the Related Art Precisio is a conventional capacitive sensor.
n Accelerometers with μg Resol-ution, Sensors and
Actuators, A21-A23 (1990) P297-3
02 describes a capacitive acceleration sensor.

[0003]

In the capacitive sensor, in order to apply it to an application requiring high reliability, such as an air bag sensor, it is necessary to provide the sensor with a failure diagnosis function. However, the above-mentioned prior art does not consider the failure diagnosis function. An object of the present invention is to provide a reliable capacitive sensor having a failure diagnosis function.

[0004]

To achieve the above object, a diagnostic signal is applied between the movable electrode and the fixed electrode to generate an electrostatic force between both electrodes. This electrostatic force displaces the movable electrode and changes the electrostatic capacitance between the movable electrode and the fixed electrode. By observing the change of the electrostatic capacity by the capacity detecting means, it is possible to determine the failure of the capacitive sensor.

Further, a diagnostic signal is applied between the movable electrode and the fixed electrode, and the leak current flowing between both electrodes is measured. If the value of this leak current exceeds the specified range, it can be determined that the capacitive sensor is out of order.

[0006]

When a diagnostic signal is applied between the movable electrode and the fixed electrode, an electrostatic force acts between both electrodes to move the movable electrode. The movement of the movable electrode is detected by the capacitance detecting means,
By observing this output, it is possible to check the failure of the capacitive sensor by checking whether or not a predetermined amount of change is obtained.

When a voltage is applied between the movable electrode and the fixed electrode, a leak current flows between both electrodes. This leak current is measured, and if it exceeds the specified range, it can be judged as a failure.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description, the detection principle of a capacitive acceleration sensor will be described with reference to FIG. The detection unit 10 includes a movable electrode 13 supported by a beam 14 and having a function of a weight, and two fixed electrodes 11 and 12 facing the movable electrode 13. When vertical acceleration acts on the detection unit 10, an inertial force proportional to the product of the mass of the movable electrode 13 and the acceleration acts on the movable electrode 13 to move the movable electrode 13 in the vertical direction.
Further, in proportion to the displacement of the movable electrode 13, a reaction force is exerted on the movable electrode 13 from the beam 14 so that the movable electrode 13 is returned to its original position. Therefore, the movable electrode 13 is displaced (displacement in which the inertial force and the drag force are balanced) according to the acceleration. Since the gap between the movable electrode 13 and the fixed electrode 11 changes due to the displacement of the movable electrode 13, the movable electrode 13 and the fixed electrode 11 change.
The capacitance C ₁ between them changes. Similarly, the electrostatic capacitance C ₂ between the movable electrode 13 and the fixed electrode 12 also changes. Therefore, the capacitance C ₂ between the capacitance C ₁ and the movable electrode 13 between the fixed electrode 11 and movable electrode 13 fixed electrode 12 in response to an acceleration in response to acceleration applied to the detection section 10 as shown in FIG. 3 Changes to. An output based on the acceleration by detecting the difference of the capacitances C _1, C ₂ the change in capacitances C _1, C ₂ by the electrostatic capacitance detection signal generator 20 and the capacitance detector 21 Obtainable.

Next, the principle and operation of detecting the capacitance will be described with reference to FIGS.

As shown in FIG. 4, the capacitance detector 21 is composed of an operational amplifier 42, an integrator with reset in which a capacitance C _F and a switch 41 are provided in a feedback portion of the operational amplifier 42, and a sample hold circuit 43. .. The inverting input of the operational amplifier 42 is connected to the movable electrode 13 of the detection unit 10. Further, rectangular waves V ₁ and V ₂ that are mutually inverted are output from the capacitance detection signal generator 20, and are connected to the fixed electrodes 12 and 13, respectively. For the sake of convenience, the detection unit 10 has a capacitance C ₁ ,
It is represented by C ₂ .

Next, the capacitance detection operation will be described with reference to the timing chart of FIG. First, the switch 41 is short-circuited by the signal φR to discharge the electrostatic capacitance C _F. At the next moment, the rectangular wave V ₁ applied to the fixed electrodes 11 and 12 falls and the rectangular wave V ₂ rises. At this time, the capacitance C ₁
Is discharged and C ₂ is charged, so that an electric charge of V _P (C ₁ -C ₂ ) flows in the electrostatic capacitance C _F. Here, V _P is the amplitude of the rectangular waves V ₁ and V ₂ . Therefore, a voltage represented by the equation (1) is generated at the output V _O of the operational amplifier 42.

[0012]

[Equation 1]

By sampling this voltage by the sample hold circuit 43, the capacitance C ₁ and the capacitance C
It is possible to obtain a voltage according to the difference of ₂ . Where (1)
As can be seen from the equation, the sensitivity of the electrostatic capacitance detector 21 is determined by the amplitude V _{P of the} rectangular waves V ₁ and V ₂ applied to the fixed electrodes 11 and 12.
And the feedback capacitance C _F , and does not depend on the DC voltage components of the rectangular waves V ₁ and V ₂ . Further, the sample-hold circuit 43 has selectivity for frequency by periodically sampling. As a result, even if the low-frequency diagnostic signal is superimposed on the rectangular waves V ₁ and V ₂ , the capacitance detector 21 is not affected.

Next, a capacitive acceleration sensor according to a first embodiment of the present invention will be described with reference to FIG. The capacitive acceleration sensor according to this embodiment includes a diagnostic control circuit 1, a capacitance detecting signal generator 2, a switch 3, 4, an electrostatic capacitance 5, 6, resistors 7, 8, a booster circuit 9, a detecting unit 10, Capacitance detector 1
5, the output adjusting circuit 16 and the switches 17 and 18, which are provided by adding a diagnostic function to the aforementioned capacitive sensor. The diagnostic functions include a leak current inspection diagnosis for measuring a leak current between the movable electrode 13 and the fixed electrodes 11 and 12, and a disconnection inspection for inspecting the wiring to the movable electrode 13 and the fixed electrodes 11 and 12. There are a diagnosis, a high acceleration electrostatic diagnosis in which the movable electrode 13 is forcibly vibrated largely by electrostatic force, and a low acceleration electrostatic diagnosis in which the movable electrode 13 is forcedly vibrated small by electrostatic force.

The execution of the diagnosis is controlled by the diagnosis control circuit 1 based on the signal of the diagnosis signal 1. FIG. 6 shows the relationship between the diagnostic signal 1 and the diagnostic content. First, when the diagnostic signal 1 is at a high level, the low acceleration electrostatic diagnosis is always executed. Further, when the diagnostic signal 1 is low level, the leakage current inspection diagnosis and the disconnection inspection diagnosis are sequentially executed, and after the disconnection diagnosis is completed, the high acceleration electrostatic diagnosis is executed while the diagnostic signal 1 is low level. In other words, the diagnostic signal 1 is set to a high level during normal measurement to constantly perform low-acceleration electrostatic diagnosis, vibrate the output with an amplitude that does not hinder the measurement of acceleration, and detect this vibration to perform diagnosis. The diagnostic signal 1 is maintained at a low level at the time of start-up or in a short time, and leak current inspection diagnosis, disconnection inspection diagnosis, and high acceleration electrostatic diagnosis can be executed to perform a comprehensive diagnosis operation. ..

Each diagnosis will be described below in sequence.

First, the leakage current inspection diagnosis will be described with reference to the leakage current inspection diagnosis timing chart of FIG. The leak current inspection diagnosis is controlled by the diagnosis control circuit 1 and is started at the fall of the diagnosis signal 1. When the leakage current inspection diagnosis starts, the signal φC goes high,
Then, the signal φMR is set to the high level. As a result, the leak current between the movable electrode 13 and the fixed electrodes 11 and 12 can be measured.

As a result, the capacitance detector 1 will be described later.
The output voltage Vo of No. 5 increases in proportion to the product of leak current and time as shown in FIG. The diagnostic control circuit 1 compares the output voltage Vo with a reference voltage after a predetermined time, and when the output voltage Vo exceeds a certain range, that is, when the leak current is larger than a specified value, the signal φoff is changed to the diagnostic signal 1. Is at a low level, that is, is kept at a low level until the high acceleration electrostatic diagnosis ends. When the signal φoff becomes low level, the switch 17 opens and the switch 18
Are short-circuited and hold the output at a constant voltage (Vcc / 2). As a result, the output is held at a constant voltage during the high acceleration electrostatic diagnosis described later. Therefore, if the acceleration sensor according to the present embodiment is normal, the acceleration sensor according to the present embodiment detects that the acceleration sensor is abnormal by keeping a place where the output should vibrate by the high acceleration electrostatic diagnosis. You can inform the system side to be used.

Next, the principle of measuring the leak current will be described with reference to the structure of the capacitance detector 15 shown in FIG. 8 and the timing chart for leak current inspection diagnosis shown in FIG. The capacitance detector 15 has a logic gate 81 added to the capacitance detector 21 so that the switch 41 can be held in an open state by a signal φMR. That is, the signal φC and the signal φ
By setting MR to high level, fixed electrodes 11 and 1
By connecting 2 to the power supply Vcc and opening the switch 41, the feedback capacitance C _F is charged with the leak current flowing from the power supply Vcc to the movable electrode 13 via the fixed electrodes 11 and 12. As a result, the output Vo of the capacitance detector 15
As shown in FIG. 9, a voltage proportional to the product of the leak current and the time is generated. By observing this voltage after a fixed time, the leak current between the movable electrode 13 and the fixed electrodes 11 and 12 can be measured.

Next, the disconnection inspection diagnosis will be described. The disconnection inspection diagnosis is a diagnosis for inspecting the presence or absence of disconnection by measuring the sum of the electrostatic capacitances C ₁ and C ₂ between the movable electrode 13 and the fixed electrodes 11 and 12. For example, when the wiring of the movable electrode 13 is broken, the sum of the electrostatic capacitances C ₁ and C ₂ between the movable electrode 13 and the fixed electrodes 11 and 12 becomes zero. In addition, the fixed electrode 1
When the wiring of 1 or the fixed electrode 12 is broken, it is reduced to half as compared with when it is not broken. That is, the electrostatic capacitance C between the movable electrode 13 and the fixed electrodes 11 and 12
By measuring the sum of ₁ and C ₂ , the presence or absence of disconnection can be inspected.

Next, the capacitance C between the movable electrode 13 and the fixed electrodes 11 and 12 will be described with reference to the configuration of the capacitance detection signal generator 2 of FIG. 10 and the timing chart of the capacitance detection signal generator 2. _The method of measuring the sum of ₁ and C ₂ will be described.
The capacitance detection signal generator 2 includes an oscillator 101 and an inverter 1.
02, 103, 104, 107, 108 and switch 1
It is composed of 05 and 106. As shown in FIG. 11, the capacitance detection signal generator 2 outputs mutually inverted rectangular waves V ₁ and V ₂ when the signal φF is at the low level, and outputs the same phase when the signal φF is at the high level. It outputs rectangular waves V ₁ and V ₂ . That is, when the signal φF is at a low level and the rectangular waves V ₁ and V ₂ are rectangular waves in which they are mutually inverted, the capacitance detector 15 has the electrostatic capacitance C ₁ between the movable electrode 13 and the fixed electrodes 11 and 12 as described above. When the signal φF is at a high level and the rectangular waves V ₁ and V ₂ are rectangular waves of the same phase, a voltage proportional to the difference between C ₂ is output, and the capacitance detector 15 operates between the movable electrode 13 and the fixed electrodes 11 and 12. It outputs a voltage proportional to the sum of the capacitances C ₁ and C ₂ . Therefore, the sum of the electrostatic capacitances C ₁ and C ₂ between the movable electrode 13 and the fixed electrodes 11 and 12 can be detected by setting the signal φF to the high level.

Next, the operation of the disconnection inspection diagnosis will be described with reference to the timing chart at the time of the disconnection inspection diagnosis in FIG. The disconnection inspection diagnosis is controlled by the diagnosis control circuit 1 and is started after the leakage current inspection diagnosis is completed, that is, when the signal φMR falls. When the disconnection inspection diagnosis is started, the signal φF is set to a high level, the rectangular waves V ₁ and V ₂ applied to the fixed electrodes 11 and 12 are set to signals in phase, and the electrostatic capacitance C between the movable electrode 13 and the fixed electrodes 11 and 12 is set. A voltage Vo proportional to the sum of ₁ and C ₂ is output to the capacitance detector 15. This output voltage Vo is compared with a reference voltage by the diagnostic control circuit 1, and when this output voltage Vo exceeds a certain range, that is, when the sum of the electrostatic capacitances C ₁ and C ₂ deviates from a specified value. Holds the signal φoff at the low level while the diagnostic signal 1 is at the low level. As a result, by holding the output at a constant voltage as in the leak current inspection diagnosis, it is possible to inform the system using the acceleration sensor that the acceleration sensor of this embodiment is abnormal.

Next, the high acceleration electrostatic diagnosis will be described with reference to the timing chart in the high acceleration electrostatic diagnosis of FIG. The high acceleration electrostatic diagnosis is controlled by the electrostatic diagnosis control circuit 1, and is executed while the diagnostic signal 1 is at a low level after the disconnection inspection diagnosis is completed. During the high acceleration electrostatic diagnosis, the booster circuit 9 alternately outputs a high voltage to V _C1 and V _C2 according to the diagnostic signal 2. That is, when the diagnostic signal 2 is at a high level, V _C1 is maintained at a high voltage, and V _C2 has a half of the amplitude V _P of the rectangular waves V ₁ and V ₂ generated from the capacitance detection signal generator 1. When a diagnostic signal 2 is at a low level, V _C1 is held at a voltage half the amplitude V _P , and a high voltage is output at V _C2 . The reason why the voltage of half the amplitude V _P is output is that the electrostatic force acting on the movable electrode 13 by the outputs V ₁ and V ₂ of the electrostatic capacity detection signal generator is the lowest. The magnitude of this high voltage is a voltage that causes the movable electrode 13 to generate an acceleration of 10% to 90% of the full scale of the acceleration sensor.

The high voltage generated by this boosting circuit is alternately applied to the fixed electrodes 11 and 12 via the resistors 7 and 8 as shown in FIG. 13, and the movable electrode 13 is fixed to the fixed electrodes 11 and 1.
Move to 2 side alternately. As a result, the electrostatic capacitance between the movable electrode 13 and the fixed electrodes 11 and 12 changes, and the capacitance detector 1
The output of the electrostatic capacitance detector 15 is changed according to the diagnostic signal 2 by being reflected in the output of No. 5. By detecting this change with a system that inputs the output of the acceleration sensor, it is possible to determine whether or not the acceleration sensor has a failure. The frequency of the diagnostic signal 2 is set to be sufficiently lower than the cutoff frequency determined by the electrostatic capacitances 5, 6, and the resistors 7, 8. In addition, the output V of the capacitance detection signal generator 2
The frequencies of ₁ and V ₂ are set to be sufficiently higher than the cut-off frequency determined by the capacitances 5, 6, and resistors 7, 8. This prevents the capacitance detection and the electrostatic diagnosis from interfering with each other.

Next, the low acceleration electrostatic diagnosis will be described with reference to the timing chart of the low acceleration electrostatic diagnosis shown in FIG. The low acceleration electrostatic diagnosis is constantly performed while the diagnostic signal 1 is at a high level. During the low acceleration electrostatic diagnosis, a high voltage (voltage for generating an electrostatic force equivalent to an acceleration of 5% or less of the full scale of the acceleration sensor or less) is alternately applied to V _C1 and V _C2 from the booster circuit 9 according to the diagnostic signal 2. ) Is output. The high voltage generated by this booster circuit is passed through resistors 7 and 8,
As shown in FIG. 14, the voltage is alternately applied to the fixed electrodes 11 and 12, and the movable electrode 13 is moved to the fixed electrodes 11 and 12 side alternately. As a result, the movable electrode 13 and the fixed electrodes 11 and 12
The capacitance between them changes and is reflected in the output of the capacitance detector 15, and the output of the capacitance detector 15 changes according to the diagnostic signal 2. By detecting this change with a system that inputs the output of the acceleration sensor, it is possible to determine whether or not the acceleration sensor has a failure.

Next, the booster circuit 9 will be described with reference to the configuration of the booster circuit of FIG. 15 and the timing chart of the booster circuit of FIG. The booster circuit 9 includes switches 151, 152,
155, 156, 159, 160, diodes 153, 15
7, 161, 163, capacitance 154, 158, 16
2, 164 and a switching circuit 165. Switches 151, 152, 155, 156, 159, 16
0 is the signals φ1, φ2, φ3, φ4, φ5 shown in FIG.
It is opened and closed by φ6 and operates by repeating states S1, S2, S3, and S4. First, in the state S1, the switch 152 is short-circuited, and the electrostatic capacitance 154 becomes the diode 1
It is connected to the power source Vcc via 53. That is, the electrostatic capacity 154 is charged with the power supply voltage. State S2, S3, S
4, the switch 151 is short-circuited, and the potential of the electrostatic capacitance 154 on the switch 151 side becomes the power supply voltage Vcc. Therefore, the voltage V1 is the sum of the power supply voltage Vcc and the power supply voltage Vcc charged in the electrostatic capacitance 154, that is, the power supply voltage is doubled. Further, since the switch 156 is short-circuited in the state S2, the electrostatic capacity 158 is charged with the voltage V1 (double power supply voltage). In the states S3 and S4, the switch 159 is short-circuited, so that the voltage V2 is the sum of the voltage V1 and the voltage charged in the electrostatic capacitance 158, that is, four times the power supply voltage is generated. Further, in the state 3, since the switch 160 is short-circuited, the capacitance 1
A voltage V2 (four times the power supply voltage) is charged in 62. In the state S4, since the switch 159 is short-circuited, the voltage V3 is the sum of the voltage V2 and the voltage charged in the electrostatic capacity 162, that is, the power supply voltage is eight times as high. By holding this eightfold power supply voltage by the peak detector circuit composed of the diode 163 and the electrostatic capacitance 164, it is possible to obtain eightfold power supply voltage as the DC voltage for the voltage V4. This power supply voltage is boosted by a factor of 8 to switch the voltage to the switching circuit 165.
The signals V _C1 and V _C2 can be generated by switching the signals.

Here, the advantages of the booster circuit will be briefly described. This booster circuit increases the voltage by 2 times, 4 times, 8 times, 1
It can be boosted up to 6 times. Therefore, it is possible to reduce the number of boosting stages as compared with the generally used Cock-Cloft circuit. This is very useful in integrating the booster circuit. In an integrated circuit, the loss per stage of the booster circuit is very large due to the influence of parasitic elements. Therefore, when the 8 × booster circuit is composed of the Cock-Cloft circuit, the number of stages is required to be 15 or more, and a very large chip area is required. However, in this booster circuit, the number of boosting stages can be reduced, so that the chip area can be reduced.

Next, a failure detection system using the acceleration sensor of the first embodiment will be described. FIG. 17 shows the configuration of this system. This system includes an acceleration sensor 171 and a microcomputer 172. The microcomputer 172 has a function of inputting the sensor output of the acceleration sensor 171 and diagnostic signals 1 and 2.
And has a function of controlling the diagnostic operation of the acceleration sensor 171. The present system can be applied to a system that requires diagnosis of a sensor such as an air bag sensor.

Next, FIG. 18 shows a flowchart of the failure detection system, and the operation of this system will be described.
The processing of this system includes initial diagnosis processing, constant diagnosis processing, and failure output processing. First, the initial diagnosis process will be described. The initial diagnosis processing is S1 to S in the flowchart of FIG.
It is represented by 4. The contents of each step of S1 to S4 will be described below. In S1, the diagnostic signal 1 is set to the low level, and the acceleration sensor is made to sequentially execute the leak current inspection diagnosis, the disconnection inspection diagnosis, and the high acceleration electrostatic diagnosis. In S2, the diagnostic signal 2 is set to low level, and the sensor output is read into the variable V1. S3
Then, the diagnostic signal 2 is set to the high level and the sensor output is set to the variable V.
Read in 2. In S4, the difference between V1 and V2 is obtained, and it is determined whether the difference is equal to or larger than the reference value. Next, the constant diagnosis process will be described. The continuous diagnosis process is S5 to S5 in the flowchart of FIG.
The infinite loop represented by S9 is continued until the power is turned off. The contents of each step of S5 to S9 will be described below.
In step S5, the diagnostic signal 1 is set to a high level, and the acceleration sensor is subjected to low acceleration electrostatic diagnosis and variables are initialized.

In S6, the diagnostic signal 2 is set to low level, and the sensor output is read into the variable V1. In S7, the diagnostic signal 2 is set to high level, and the sensor output is read into the variable V2. S
In FIG. 8, V1 and V2 filtering processing is performed. This is because the change in V1 and V2 is small and the S / N ratio is poor, so that this is improved. In S9, the difference between the values VS1 and VS2 obtained by filtering V1 and V2 is obtained, and it is determined whether the difference is equal to or larger than the reference value.

Next, a capacitive acceleration sensor according to a second embodiment of the present invention will be described. FIG. 19 shows the configuration of this embodiment. In the first embodiment, in the high acceleration electrostatic diagnosis and the low acceleration electrostatic diagnosis, V _C1 and V _C2 are vibrated in synchronization with the diagnostic signal 2. The present embodiment is an acceleration sensor in which V _C1 and V _C2 are vibrated by an oscillator built in the diagnostic control circuit 192 without using the diagnostic signal 2. Since the oscillator is built in, it is necessary to output information about V _C1 and V _C2 to the system using this acceleration sensor during high acceleration electrostatic diagnosis and low acceleration electrostatic diagnosis. Therefore, in this embodiment, an event signal and a sync signal as shown in FIG. 20 are provided. First, the event signal was supplied to the system side as an interrupt signal. That is, the event signal is a signal that becomes low level for a short time in synchronization with the output change as shown in FIG. 20 at the time of high acceleration diagnosis or low acceleration diagnosis. The synchronization signal was supplied to the system side as a reference signal for synchronous detection. That is, the output of the sync signal changes in synchronization with the high-acceleration diagnosis or low-acceleration diagnosis whose output changes as shown in FIG. In the system using the acceleration sensor of the present embodiment, a method that does not use the event signal or the synchronization signal is also possible. This is because the oscillator of the diagnostic control circuit 1 operates in synchronization with the falling edge of the diagnostic signal 1. Therefore, the system side can know the content of the diagnosis of the acceleration sensor by managing the time after setting the diagnosis signal 1 to the low level.

Next, a capacitive pressure sensor according to a third embodiment of the present invention will be described. FIG. 21 shows the configuration of this embodiment. Prior to the description, the detection principle of the capacitive pressure sensor will be described. The detection unit 219 includes the glass substrate 217 and the glass substrate 2
It is composed of a fixed electrode 218 and a silicon diaphragm 220 which are vapor-deposited on 17. When pressure is applied to the silicon diaphragm 220, the silicon diaphragm 220 and the fixed electrode 21
8 changes, and the capacitance between the silicon diaphragm 220 and the fixed electrode 318 changes. The capacitance detector 221 detects this change in electrostatic capacitance, and the output adjustment circuit 22
By adjusting the output with 2, the output voltage according to the pressure can be obtained.

Also in this embodiment, the diagnostic control circuit 211,
Capacitance detection signal generator 212, switch 213, capacitance 214, resistor 215, booster circuit 216, capacitance detector 221, output adjustment circuit 222, switch 22.
3, 224, and has leak current inspection diagnosis, high acceleration electrostatic diagnosis, and low acceleration electrostatic diagnosis. First, in the leak test inspection diagnosis, the signal φC and the signal φMR are set to a high level to generate an output proportional to the product of the leak current and the time at the output of the electrostatic capacitance detector 221, and the diagnostic control circuit 211 outputs this voltage. If the comparison is made and the value exceeds the reference value, the signal φoff is set to the low level, and the output is held at a constant voltage. The high acceleration electrostatic diagnosis and the low acceleration electrostatic diagnosis can be achieved by applying the high voltage generated in the booster circuit 216 to the fixed electrode 217 via the resistor 215, similarly to the first embodiment. The diagnostic operation sequence is the same as in the first embodiment.

[0034]

As described above, according to the present invention, since it is possible to diagnose a failure of the capacitive sensor, the reliability of the system using the capacitive sensor can be improved.

[Brief description of drawings]

FIG. 1 is a configuration of a first embodiment.

FIG. 2 is a configuration of a conventional acceleration sensor.

FIG. 3 shows changes in capacitance with respect to acceleration.

FIG. 4 is a configuration of a capacitance detector.

FIG. 5 is a timing chart of a capacitance detector.

FIG. 6 shows the relationship between diagnostic signal 1 and diagnostic content.

FIG. 7 is a timing chart at the time of leakage current inspection diagnosis.

FIG. 8 is a configuration of a capacitance detector.

FIG. 9 is a timing chart at the time of leak current inspection diagnosis.

FIG. 10 is a configuration of a capacitance detection signal generator.

FIG. 11 is a timing chart of a capacitance detection signal generator.

FIG. 12 is a timing chart during disconnection inspection diagnosis.

FIG. 13 is a timing chart at the time of high acceleration electrostatic diagnosis.

FIG. 14 is a timing chart at the time of low acceleration electrostatic diagnosis.

FIG. 15 shows a configuration of a booster circuit.

FIG. 16 is a timing chart of a booster circuit.

FIG. 17 is a configuration of a failure detection system using the acceleration sensor according to the first embodiment.

FIG. 18 is a flowchart of a failure detection system.

FIG. 19 is a configuration of the second embodiment.

FIG. 20 is a timing chart of a diagnostic control signal.

FIG. 21 is a configuration of the third embodiment.

[Explanation of symbols]

1 ... Diagnostic control circuit, 2 ... Capacitance detection signal generator, 3
... switch, 4 ... switch, 5 ... electrostatic capacity, 6 ... electrostatic capacity, 7 ... resistor, 8 ... resistor, 9 ... booster circuit, 10 ... detection unit, 11 ... fixed electrode, 12 ... fixed electrode, 13 ... movable electrode, 14 ... beam, 15 ... electrostatic capacity detector, 16 ... output adjusting circuit, 17 ... switch, 18 ... switch, 20 ... electrostatic capacity detection signal generator, 21 ... electrostatic capacity detector, 41
... switch, 42 ... operational amplifier, 43 ... sample-hold amplifier, 81 ... logic gate, 101 ... oscillator, 102
... Inverter, 103 ... Inverter, 104 ... Inverter, 105 ...
Switch, 106 ... Switch, 107 ... Inverter, 108
... Inverter, 151 ... Switch, 152 ... Switch, 15
3 ... Diode, 154 ... Capacitance, 155 ... Switch, 156 ... Switch, 157 ... Diode, 158 ...
Capacitance, 159 ... Switch, 160 ... Switch, 16
1 ... Diode, 162 ... Electrostatic capacity, 163 ... Diode, 164 ... Electrostatic capacity, 165 ... Switching circuit, 171
... acceleration sensor, 172 ... microcomputer, 191 ... diagnostic control circuit, 211 ... diagnostic control circuit, 212 ... electrostatic capacitance detection signal generator, 213 ... switch, 214 ... electrostatic capacitance, 2
15 ... Resistor, 216 ... Step-up circuit, 217 ... Glass substrate, 218 ... Fixed electrode, 219 ... Detection part, 220 ... Silicon diaphragm, 221 ... Capacitance detector, 222 ...
Output adjustment circuit, 223 ... Switch, 224 ... Switch.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Kuragaki 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture, Hitachi Research Laboratory, Hitachi Ltd. Automotive Equipment Division

Claims (6)

[Claims] 1. A capacitive sensor having a movable electrode whose position changes in response to an object to be detected and a plurality of fixed electrodes arranged facing the movable electrode, wherein static electricity is provided between the fixed electrode and the movable electrode. A capacitive sensor having means for generating force. 2. A capacitive sensor having a movable electrode whose position changes in response to an object to be detected and a plurality of fixed electrodes arranged so as to face the movable electrode, and a relay between the fixed electrode and the movable electrode. A capacitive sensor having a means for measuring an electric current. 3. A capacitive sensor having a movable electrode whose position changes in response to a detection target and a plurality of fixed electrodes arranged facing the movable electrode, wherein an amplifier, an electrostatic capacitance, and the electrostatic capacitance are provided. A capacitive sensor comprising: a capacitance detecting means having a switch for discharging the battery; and a means for holding the switch in an open state. 4. A capacitive sensor having a movable electrode whose position changes in response to an object to be detected and a plurality of fixed electrodes arranged facing the movable electrode, and means for vibrating the output of the capacitive sensor. A capacitive sensor having: 5. A control system having a signal relating to acceleration as an input, comprising control means for judging that said signal is a normal signal when said input signal is vibrating. 6. A capacitive sensor having a movable electrode whose position changes in response to a detection target and a plurality of fixed electrodes arranged facing the movable electrode, wherein the movable electrode and the plurality of fixed electrodes are provided. A capacitive sensor having means for measuring the total sum of electrostatic capacitances between the two.